As a power supply frequency, 50 Hz and 60 Hz are used worldwide. Accordingly, some countries and regions have different power supply frequencies. In particular, Japan is divided into two regions. When a fluorescent lamp without an inverter function is used as a light fixture, the fluorescent lamp makes cyclic turning on and off 100 times per second in a 50 Hz region of the power supply frequency, and 120 times per second in a 60 Hz region of the power supply frequency.
When using a digital video camera or digital still camera including a solid-state image pickup device such as a CCD or CMOS sensor under fluorescent lighting, the amount of charge stored in the solid-state image pickup device can change every image read timing even though the solid-state image pickup device is set at the same exposure time because of the relationship between the image read timing (shutter speed) of the solid-state image pickup device and the blinking cycle of the fluorescent lamp. If the amount of charge stored in the solid-state image pickup device changes, the brightness of the image vary, which causes flickering like blinking of a video screen.
As a method of suppressing such flicker, a technique is known which is disclosed in Japanese patent application laid-open No. 10-257381/1998, for example. In the conventional technique, as for the input video signal of one field supplied from a CCD, its image is divided into a plurality of blocks in the horizontal direction, and average values of the luminance of individual blocks are calculated. Subsequently, average values that give a maximum value and a minimum value are calculated from the average values of the luminance calculated, and from the maximum value and minimum value of the luminance a correction coefficient for correcting the gradation of every field is calculated. In response to the luminance fluctuations in the input video signal, that is, to the cycle of the flicker, the gradation (luminance value) of the input video signal is corrected using the correction coefficient obtained one cycle before.
The conventional technique described in the foregoing document employs a method of calculating the average values of the luminance. Thus, it is a digital gain integrating method using only the luminance as an item to be calculated. However, since the ranges of the pixel values, which fluctuate because of the flicker, differ in individual color components constituting the image, the foregoing method cannot suppress the flicker accurately.
In addition, the power supply frequency has a minute error in practice, and sometimes deviates from an exact value of 50 Hz or 60 Hz. When the power supply frequency has a minute error, the method described in the foregoing document, which carries out correction according to the data one cycle before the flicker, cannot suppress the flicker accurately because the pixel signal one cycle before is not always equal to the pixel signal in the next cycle.
Furthermore, when the pixel values fluctuate because of external factors such as changes of an image pickup object, the integral of the digital gain based on the pixel values offers a problem of amplifying the flicker.
Moreover, the flicker elimination using the digital gain has a problem of reducing the S/N ratio and deteriorating the image quality because the digital gain is applied to all the pixels.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide an image pickup apparatus capable of suppressing the flicker accurately without fail when applied to the digital color image pickup equipment such as a digital video camera and digital still camera.